Syrian refugees chants anti-President Bashar Assad slogans and demand an international military strike on the Syrian regime at Zaatari Syrian refugee camp, in Mafraq, Jordan, Wednesday, Aug. 28, 2013. A week after the purported chemical attack on rebel-held areas outside Damascus, momentum has been building for a possible strike by the U.S. and its allies against the regime of Syrian President Bashar Assad. The Associated Press

DAMASCUS, Syria — Fears of a possible U.S. strike against Syria’s regime over an alleged chemical weapons attack rippled across the region Wednesday, as about 6,000 Syrians fled to neighboring Lebanon in a 24-hour period and Israelis scrambled for gas masks in case Damascus retaliates against them.

U.N. chief Ban Ki-moon pleaded for more time for diplomacy and to allow U.N. investigators to complete their work. The experts, wearing flak jackets and helmets, collected blood and urine samples from victims during a visit to at least one of the areas hit in last week’s attack.

Seven days after chemical weapons were purportedly unleashed on rebel-held suburbs of the Syrian capital, momentum grew toward Western military action against President Bashar Assad’s regime. At the same time, Syria’s chief allies, Russia and Iran, warned of dire consequences for the region if any armed intervention is undertaken.

U.S. leaders, including Vice President Joe Biden, have charged that Assad’s government was behind the Aug. 21 attack that Doctors Without Borders says killed at least 355 people. The White House says it’s planning a possible military response while seeking support from international partners.

The U.S. has not presented concrete proof of Syrian regime involvement in the attack, and U.N. inspectors have not endorsed the allegations, although the U.N. envoy to Syria, Lakhdar Brahimi, said evidence suggests some kind of “substance” was used that killed hundreds.

Two senior Obama administration officials said U.S. intelligence agencies are drawing up a report laying out the evidence against Assad’s government. The classified version would be sent to key members of Congress and a declassified version would be made public.

One of the officials said the administration is considering more than a single set of military strikes and “the options are not limited just to one day” of assault.

“If there is action taken, it must be clearly defined what the objective is and why” and based on “clear facts,” the senior administration official said on condition of anonymity because he wasn’t authorized to discuss internal deliberations publicly.

President Barack Obama is weighing a limited response that focuses on punishing the Syrian government for violating international agreements that bar the use of chemical weapons. Any U.S. military action, officials say, would not be aimed at toppling the Assad regime or vastly altering the course of Syria’s civil war, which has already claimed 100,000 dead.

As the U.S., France and Britain push for military action, the U.N. secretary-general urged restraint to give U.N. inspectors time to finish their investigation, which began Monday.

“Let them conclude ... their work for four days and then we will have to analyze scientifically” their findings and send a report to the U.N. Security Council, Ban said. The U.N. said the analysis would be done “as quickly as possible.”

Syria’s Ambassador to the U.N., Bashar Ja’afari, said he sent Ban a letter demanding that the inspectors extend their investigation to what he described as three chemical weapons attacks against Syrian soldiers in the Damascus suburbs. He said the attacks occurred on Aug. 22, 24, and 25, and that dozens of Syrian soldiers are current being treated for inhaling nerve gases.

Ja’afari also blamed the rebels for any chemical weapons attack, saying “the Syrian government is innocent of these allegations.”

Ban pleaded for more time to give diplomacy another chance to end Syria’s conflict. Marking the centenary of a venue for peaceful conflict resolution in The Hague, Netherlands, he said: “Here in the Peace Palace, let us say: Give peace a chance. Give diplomacy a chance. Stop fighting and start talking.”

But with many seeing Western intervention no longer a question of if but when, there were signs of growing fears across the wider region.

At least 6,000 Syrians crossed into Lebanon in a 24-hour period through the main Masnaa crossing, including an estimated 4,000 on Wednesday, according to Lebanese security officials in the country’s Bekaa Valley near the border. The normal daily influx is 500 to 1,000 refugees, depending on the level of fighting. The officials spoke on condition of anonymity in line with regulations.

Long lines of packed cars — some with suitcases strapped to roofs — were backed up at the frontier post, witnesses said. A security official said about 2,000 also crossed into Syria, but many of them said they were going in to evacuate relatives.

One woman, Um Ahmad, entered Lebanon with her five children, saying she fearing U.S. strikes on Damascus.

“Isn’t it enough, all the violence and fighting that we already have in the country, now America wants to bomb us, too?” the 45-year-old said, declining to give her full name for security concerns.

Her husband said they know no one in Lebanon but came anyway because of their children. “What will we do here, where will we go? I don’t know, but hopefully we’ll be safe,” he added.

Nearly 2 million Syrians have fled the country since the crisis began in March 2011, and millions more are displaced inside Syria.

Effects were also evident in Israel, where large crowds lined up at gas-mask distribution centers. Maya Avishai of the Israeli postal service, which oversees gas mask distribution, said demand has tripled in recent days. About 5 million Israelis, roughly 60 percent of the population, now have gas masks, she said.

The Israeli government ordered a “limited” call-up of reserve units to bolster civil defense preparations and to operate air-defense units near the border. Officials said the call-up is anticipated to bring in hundreds of troops.

While Israeli officials believe the chances of a Syrian strike remain slim, there are concerns that Damascus may respond to any U.S.-led military action by attacking the Jewish state, a close American ally.

On Wednesday, the U.N. inspectors visited the eastern Damascus suburbs of Mleeha and Zamalka, activists said. Amateur video showed a convoy of five cars with U.N. markings, followed by armed rebels in pickups.

The video showed the inspectors visiting a clinic and interviewing a man through a translator. Two inspectors were present as a nurse drew blood from a man on an examination table. One of the experts was heard in the video saying he and his team have collected blood, urine and hair samples.

The videos appeared consistent with other reporting by The Associated Press, including Skype interviews with anti-regime activists.

One activist said the team took hair and skin samples of five suspected victims in Zamalka during a 90-minute visit. He spoke on condition of anonymity for fear of regime reprisals.

At the U.N., the five permanent members of the Security Council failed to reach an agreement on a British-proposed resolution that would authorize the use of military force against Syria.

The draft resolution — were it to be put to a vote — would almost certainly be vetoed by Syria ally Russia as well as China, which have blocked past attempts to sanction the Assad regime. The document was being sent back to governments for consultations, according to a Western diplomat, who spoke on condition of anonymity because the discussions were private and he was not authorized to speak to reporters.

A French diplomatic official acknowledged that the British resolution has virtually no chance of passing, but is being introduced to show that all diplomatic steps were being exhausted. He spoke on condition of anonymity because he was not allowed to disclose details of the deliberations.

French President Francois Hollande convened his top defense advisers about Syria, and was to meet Thursday with the head of Syria’s main opposition group.

In London, Prime Minister David Cameron held a meeting on Syria and said the military and security chiefs at Britain’s National Security Council “unanimously” backed his call for action. Parliament was expected to convene Thursday to discuss the matter and possibly vote on whether Britain would participate.

Jordan, meanwhile, said it will not be used as a launching pad for attacks on Syria and favors a diplomatic solution. A U.S.-led strike would involve cruise missile attacks from the sea, which would not need to cross or make use of Jordanian territory.

The remarks underlined the U.S. ally’s efforts to avoid further friction with its larger neighbor for fear that Assad or his Iranian backers could retaliate.

Two of Syria’s staunchest backers, Iran and Russia, warned that an attack by U.S. and its allies against Syria would set the region alight.

Such strikes “will lead to the long-term destabilization of the situation in the country and the region,” said Russian Foreign Minister Sergey Lavrov.

“Intervention of foreign and extra-regional powers in a country has no result other than sparking fire,” Iran’s state TV quoted Khamenei as saying. “Waging a war is like a spark in a gunpowder store ... its dimensions and consequences can’t be predicted.”

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